Various methods are known for removing NO.sub.x from the NO.sub.x -containing exhaust gases such as released combustion gas, and among such known methods, the so-called catalytic reduction method using ammonia as reducer is credited with high utility for its unchanged effectiveness even in the treatment of discharge gases with relatively high concentrations of NO.sub.x and/or oxygen. According to this method, the NO.sub.x -containing gas is contacted with ammonia in the presence of a catalyst at a catalyst-activated temperature to effect a reaction between NO.sub.x and ammonia to thereby transform NO.sub.x into N.sub.2. The temperature at which the catalyst is activated varies depending on the kind of the catalyst used. Although many of the catalysts of the type usable in this method become active at a temperature of 200.degree. to 400.degree. C., there are some which show a very high activity at a relatively low temperature, such as 100.degree. to 230.degree. C. From the industrial viewpoint, it is preferred to practice the method by using a catalyst which is activated at a low temperature.
Generally, when a NO.sub.x -containing discharge gas is contacted with ammonia to let NO.sub.x react with ammonia, there is by-produced ammonium nitrate. This ammonium nitrate is thermally decomposed at a temperature higher than 230.degree. C., but it is scarcely decomposed at a lower temperature and deposits on the catalyst layers to badly affect the catalyst activity. Therefore, it has been hardly feasible to industrially perform the reaction at a temperature below 230.degree. C. in spite of many advantages such as smaller energy loss or reduced poisoning by SO.sub.2 when such element is contained in the discharge gas.